The invention relates to a machine tool with an automatic tool exchange. More specifically, the invention relates to so-called multiple tool automatic machining centers.
Such machine tools are known, for example, from German Pat. No. 1,552,324 corresponding to U.S. Pat. No. 3,524,248. This known machine has primarily three drawbacks. The tool magazine is mounted on the spindle unit, whereby vibrations may be transmitted to the spindle so that the machining accuracy is impaired. These vibrations result when the tool selection motion of the magazine must take place very rapidly whereby large braking and accelerative forces occur. Therefore, when a high machining accuracy is required, no rapid magazine movements should take place during the machining time. Depending on the position of the next tool to be exchanged in the spindle, a more or less large, but always disturbing idle time results. In this known machine, the grapple claw mechanism comprised two individual grapple claws driven in opposite directions so that the actual tool exchange operation also took too much time for today's requirements. Finally, a practically unjustifiable increase in cost may have been caused by the restriction on the size of the magazine by the space between the two vertical columns. Either the magazine would have to be very small or the spacing between the two columns would have to be chosen to be larger than necessary for other considerations thereby causing said cost increase.
However, the just mentioned machine tool also has essential advantages compared to other known comparable machine tools. Due to the double column frame construction, a very high machining accuracy is guaranteed, in principle, because any thermal expansion in the x-axis takes place symmetrically relative to the spindle and is thus compensated without any adverse effect. Furthermore, the working space, that is the space in front of the spindle which may be occupied by the work piece, is only minimally restricted, because the tool in its initial exchange position is located completely behind the front contour of the entire machine, so that the work piece may project, theoretically in any case, up to the spindle unit or rather to the column until shortly after the start of the actual tool exchange operation, namely, practically in the entire outer field or zone of the spindle. Finally, the tool exchange may take place in any position of the spindle unit, even in the lowest position, which achieves substantial time savings. Also, it is possible, by these means, to nullify the positioning tolerance when multiple machining operations take place in the same position, which is especially important for coaxial machining operations, for example for graduated borings in the work piece, because a very good coaxiality may be guaranteed in this way.
There is no prior art machine tool of a comparable type which avoids all the drawbacks of the described, known machine tool while simultaneously retaining all the advantages.
The term "machining center" has been introduced to describe machine tools of this and comparable types. A good review of the different forms of known machining centers is set forth in the periodicals "Technische Zeitschrift fuer praktische Metallbearbeitung" (Numerik) 63 (1969), Nr. 6 (2, 1969 H 6), pages 318(50) to 334(66) and "Werkstatt und Betrieb" 102, 1969, Nr. 7, pages 487 to 493. In "Numerik", page 317, right column, section 2, paragraph 1, it is explicitly pointed out that the embodiment of these processing machines in the year 1969 are very diversified despite often having identical goals or objectives. Such diversification is not present in conventional machines and is a typical indication that an ideal form or at least an optimal form of a machining center has not yet been developed. An ideal machine form is naturally understood to be a machine which avoids the disadvantages of other machines, but exhibits their advantages while an optimal machine is understood to have only those unavoidable disadvantages that must be taken into account to attain the desired advantages.
After publication of said review another series of machining centers has become known, for which these considerations apply unchanged. Thus, in the small machining center BEA 1-H, the applicant abandoned the double column frame construction method, so that the tool magazine could be constructed substantially larger. By using an angular double grapple claw, the tool exchange time has been substantially reduced as compared to oppositely moving individual grapple claws.
Simultaneously, the advantage of executing the tool exchange in each and any position of the spindle unit has been maintained.
Due to the advantages of the double column frame construction, many different machining centers have become known which were based on the underlying principle of the broad pendant construction. The tool magazine is arranged on one side of the double column frame in known machines of this type and an angular double grapple or gripping claw is provided as an exchanging mechanism (DIAG Werner Transfer-Center TCl; Deckel High Performance Machining Center S500). The spindle unit must move into a tool exchange position in this type of construction so that the advantages mentioned above must be relinquished, namely time savings due to tool exchange in each spindle position and the elimination of the positioning tolerance. Additionally, the exchange position must be located in the upper range of the y-direction displacement of the main spindle so that long tools, which necessarily tip downwardly upon insertion or removal out of the spindle by the angular double grapple claw, may by-pass the work piece table, so that in many cases traversing into the exchange position and back to the machining position requires a considerable time.
In another known machining center with a double column frame, please see German Patent Publication No. 2,808,796, the traverse path is shortened somewhat by the use of a single double grapple claw, but at the expense of an involved grapple claw construction in which the complete exchange operation takes up a considerable amount of time. In addition, the tool projects into the work space when it is in a position in which it can be grabbed by the double grapple claw.
The tool in another known machining center (Kearney and Trecker Marvin Ltd. KTM400) with a double column frame, is also brought into an exchange position corresponding to the uppermost position of the spindle unit by means of two intermediate grapple claws, and the tool is exchanged into the spindle by means of a single double grapple claw out of said uppermost position. This type of structure has the disadvantages of locating the tool exchange position at the upper end of the y-displacement and of a single double grapple claw having an involved claw construction.
Another known machining center with a double column frame is also provided with a single double grapple claw. The magazine is secured on the upper side of the double column, please see German Patent Publication No. 2,001,871. The tool spindle must also move into an exchange position in the upper range of the y-displacement in this machining center. Moreover, the tools in this known machining center project into the work space at all times and not only during the initial exchange position, whereby naturally it must be taken into account that the work space may easily extend upwardly above the machine tool itself. This may occur when only the lower area of a high or long work piece must be processed or machined. The foregoing may naturally also be achieved in that larger work pieces are reclamped on the work piece support.
A further known machining center with a double column frame made by Hueller Hille, Horizontal Machining Center nb-h 80-1 and nb-h 100-1, is again provided with a single double grapple claw, but the magazine is arranged freely next to the double column. Here again, the spindle unit must be moved into an exchange position, in the middle of the y-displacement. In addition, the entire double column must traverse to the rearward end of the z-displacement, so that even in a more favorable tool exchange machining position in the y-direction, a noticeable time loss results.
German Patent Publication No. 1,949,390 corresponding to U.S. Pat. No. 3,837,070 discloses a machining center in which the double column construction has been abandoned and in which the tool magazine is arranged on one side and the spindle unit is arranged on the other side of a single standard or column. The magazine is located above the spindle and an intermediate grapple claw is provided which takes a tool, to be exchanged in the spindle, out of the magazine and brings the tool into an initial exchange position from where it can be exchanged on the tool spindle by means of a single double grapple claw. The work space next to the spindle is free in this known machining tool yet the upper end of the spindle is not free. Besides, the advantages of the double column frame construction cannot be achieved by this structure.
German Patent Publications Nos. 1,477,347 and 1,477,358 corresponding to U.S. Pat. No. 3,300,856 disclose how in processing centers with a single column the tool magazine is arranged on the spindle tool carrier on the back part of the spindle unit and the tools are transported into the initial exchange position by means of an intermediate grapple claw in the form of a double grapple claw. The tool is then exchanged directly into the spindle from the intermediate position. In these machines, the tool also projects into the work space in the initial exchange position, the advantages of the double column construction have been abandoned, and the magazine is mounted on the spindle unit with the disadvantages mentioned above.